Some conventional X-ray CT (computed tomography) systems detect X-rays radiated from an X-ray tube to a subject and transmitted through the subject with an X-ray detector, acquire projection data, then reconstruct images from the acquired projection data (for example, Patent Document 1.)
The X-ray tube includes an anode provided with a radiation surface for radiating X-rays, as well as a cathode provided with a filament. X-rays with a cone angle enlarging from the radiation surface in the rostrocaudal direction of the subject are radiated to the subject. The size of an effective focal point, which is the apparent size when seeing the radiation surface from the side on which the X-rays are radiated, is sometimes referred to as a focal point size.
The focal point size differs depending on whether seeing the radiation surface from the anode side or the cathode side. The radiation surface is structured such that the focal point size is decreased on the anode side and increased from the anode toward the cathode.
The X-ray detector comprises X-ray detection elements two-dimensionally arrayed in the rostrocaudal direction and horizontally orthogonal thereto. The X-ray detector includes a uniform type, a hybrid type, and a non-uniform type depending on the array mode.
In the uniform type of X-ray detector, the X-ray detection elements are uniformly arrayed in the rostrocaudal direction of the subject. The size of the X-ray detection element in the rostrocaudal direction is sometimes referred to as a detector size.
In the hybrid type, the X-ray detection elements of a small detector size are arrayed in a plurality of rows in the center in the rostrocaudal direction of the X-ray detector, with the X-ray detection elements of a large detector size arrayed every specific number of rows in front and back of those of the small detector size.
In the non-uniform type, the X-ray detection elements from the small detector size to the large detector size are symmetrically arrayed in front and back.
High resolution cannot be acquired upon imaging by detecting the X-rays radiated from the focal point of the small focal point size and transmitted through the subject with the X-ray detection elements of the large detector size, making it impossible to fully utilize the X-ray detection elements of the small detector size. High and low states of resolution are relatively defined in the following description.